Electro-optical differential

ABSTRACT

An electro-optical differential device for combining the angular rotation of a ship&#39;&#39;s gyrocompass with the angular rotation of a radar antenna to compensate a radar PPI display for turning movements of a ship. Two polarizing disks are placed within a beam of light, one disk connecting with the gyrocompass and the other connecting with the antenna, and are rotated relative to each other to modulate the light. Electrical signals produced upon detection of the light beam energize a synchronous motor which rotates the PPI display in synchronism with the light modulation.

United States Patent Dallard Oct. 16, 1973 [54] ELECTRO-OPTICALDIFFERENTIAL 3,435,213 3/1969 Colbow et al 250/233 X Inventor: Albert S.Ballard San Pateo, Calif. 3,526,448 9/1970 Senseney 250/233 X [7Assignee: Raytheon p y Lexington, Primary Examiner-T. H. Tubbesing Mass-Attorney-Philip J. McFarland and Joseph D. Pannone [22] Filed: May 13,1971 211 Appl. No.2 143,178 7] ABSTRACT Remed s Application Data Anelectro-optical differential device for combining [63] Continuation ofset. NO. 867,292, Oct. 17, 1969, the angular m of a s gyrocompass W theabandoned angular rotation of a radar antenna to compensate a radar PPIdisplay for turning movements of a ship. 52 US. Cl. 343/55 T, 250/233Polarizing disks are Placed Within a beam light, 51 lm. Cl G01s 9/02,GOld 5/36 disk h with the gymcompass and the 581 Field of Search 343/5R, 5 ST; with the antenna h are i 250/232, 23 3 relative to each otherto modulate the light. Electrical signals produced upon detection of thelight beam en- 56] References Cited ergize a synchronous motor whichrotates the PPI dis- UNITED STATES PATENTS play in synchronism with thelight modulation.

3,085,241 4/1963 Moore 343/5 ST 12 Claims, 3 Drawing Figures GEAR gmgSYNCHRO TRAIN 1- 3s:I I I CYCLES/COMPASS REV- 20 2s Fir-W767i: 0 1'" T26 T 3 34 I 40 5 ANTENNA l I f l I l izoRPM] l LAMP %DETECTOR AMPLIFIERl|-|- %"g$g I I l GEAR I i l l TRAIN 1 l GEAR I 72 I 52/ SW'TCH l 59TRAIN I 70 '88,? aocvcuz l I I 1 Z EZ EE E YJ L l I l l I I as: I 54 I Il 1 N24 DEFLECTION l DRIVE 1 YOKE l MOTOR ELECTRO-OPTICAL DIFFERENTIALThis application is a continuation of Ser. No. 867,292, filed Oct. 17,1969, now abandoned.

BACKGROUND OF THE INVENTION A display such as a Plan Position Indicator(PPI) is often utilized on board a vehicle such as an aircraft or ship,to show the relative bearings of objects which are scanned by a rotatingradar antenna carried by the vehicle. Such display must be generated insynchronism with the rotation of the antenna relative to the ship if allthe objects are presented in their correct relative bearings. Forexample, in a well known form of PPI used on board a ship, the displayis generated with the aid of a magnetic deflection yoke rotatablymounted about the axis of the cathode ray tube (CRT) of the display, therotation of the deflection yoke being synchronized with the rotation ofthe antenna.

It is frequently desirable to have a north stabilized display such thatthe bearings of the objects displayed on the PPI remain constantrelative to the ship as it turns about its yaw axis. To implement such adisplay, a well-known angular correction obtained from a gyrocompass iscombined, by either addition or subtraction, with the antenna azimuthangle (relative to the ships heading) to offset the effect of a turningmovement of the ship. Such angular correction is combined with theantenna azimuth angle in a manner analogous to the operation ofmechanical differential gear train.

The implementation of the angular correction has usually, in the past,been accomplished by means of a servo system employing a synchro forelectrically transmitting angle data from a ships gyrocompass, and adifferential or synchro, located at a distance from the gyrocompass, forreceiving the angle data and converting it from an electrical signal toa mechanical representation such as a shaft rotation. This angularcorrection is then combined with angle data from the antenna and appliedto rotation of the PPI deflection yoke by means of a second servosystem. It is desirable to decrease the complexity of this prior artsystem, to reduce its cost especially in commercial applications, and toincrease its accuracy and speed of response. The accuracy and speed ofresponse have, in the past, been limited by the servo response due tothe low torque of the synchro. By implementing a system in which thesynchro has a minimal external load, the speed and accuracy can beimproved to provide the desired angular correction even in an extremecase such as a small ship yawing in response to a sudden wave.

Accordingly, it is an object of the invention to provide an anglecorrection differential system of reduced complexity and cost It isfurthermore an object to provide an angle correction differential systemof increased accuracy and speed of response.

SUMMARY OF THE INVENTION In accordance with the invention a beam oflight is modulated by a pair of shuttering devices, preferably a pair ofpolarizer disks, which rotate with respect to each other, each diskhaving a light transmissivity characteristic that varies periodicallywith rotation about the axis of the disk, whereby the beam of light ismodulated at a rate proportional to the difference of the angularrotation rates of the disks. One disk is connected to a first source ofrotation (as the gyrocompass) and the second disk is connected to asecond source of rotation (as the antenna). A detector, such as aphotocell or phototransistor, intercepts the light passing through thedisks to generate a control signal for driving a rotatable output devicein synchronism with the modulation of the beam of light.

BRIEF DESCRIPTION OF THE DRAWINGS The aforementioned objects and otherfeatures of the invention are explained in the following descriptiontaken in connection with the accompanying drawings wherein:

FIG. 1 is a block diagram of the invention with mechanical connectionsindicated by dashed lines; and

FIG. 2 and FIG. 3 are plan views of alternative forms of opticalmodulation disks disclosed in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, it maybe seen that a modulator 20 (described in detail hereinafter) is drivenby a source 22, providing a ship s heading data, and a source 24,providing antenna orientation data, and produces a control signal for aload 26. The modulator 20 comprises a pair of shuttering devices such asthe two disks 28 and 30 placed transversely across a beam of light 32for varying the intensity of the beam of light 32. The beam of light 32is conveniently provided by a lamp 34 which transmits light through alens 36, positioned before disk 30, and a second lens 38, positionedbehind disk 28, which focus the beam of radiation 32 upon detector 40.Each of the disks 28 and 30 is constructed with a radiationtransmissivity characteristic that varies with rotation about the axisof the disk. Each disk 28 and 30 is preferably a polarizer such as disk42 shown in FIG. 2 or, alternatively, a shutter such as disk 44 shown inFIG. 3 having a pair of opaque segments 46 separated by a pair oftransparent segments 48. In this embodiment, the two disks 28 and 30 areconveniently mounted rotatably about their common axis AA which isapproximately parallel to the axis of the beam of light 32. Disk 28 isrotated by source 22 at a rate related to the change in ships heading,and disk 30 is rotated by source 24 at a rate related to the angularvelocity of the antenna relative to the ship. Disks 28 and 30 are ofrelatively low inertia compared to sources 22 and 24 and thereforepresent no more than a negligible loading effect on these sources forincreased accuracy and speed of response. As the two disks 28 and 30rotate relative to each other, the intensity of the light reachingdetector 40 is modulated at a rate or frequency proportional to thedifference in the angular rotation rates of the two disks. A periodicmodulation is obtained when disk 30 rotates (the rotation correspondingto the ships antenna rotation) at a constant rate and disk 28 rotates(the rotation corresponding to the ships yawing or turning) at aconstant rate or at zero angular velocity.

A well-known sinusoidal modulation is obtained when polarizing diskssuch as disk 42 are utilized. Thus, when disk 28 and disk 30 areoriented with parallel polarization, the light intensity reachingdetector 40 has a maximum intensity, and as the two disks 28 and 30rotate relative to each other to a cross polarized position, theintensity of the light reaching detector 40 drops toward zero. Atintermediary position of rotation relative to each other the two disks28 and 30 produce an intensity modulation which varies as the square ofthe cosine of the angle between the polarization directions of the twodisks 28 and 30. A similar result is obtained when disks such as thedisk 44 of FIG. 3 are utilized, in which case a triangular form ofmodulation results. Thus, as is readily seen, when the opaque segments46 of disk 28 are aligned with the transparent segments 48 of disk 30, aminimum intensity of light reaches detector 40. The intensity of thelight reaching detector 40 increases from the minimal value at a ratelinearly related to the angular orientation of the pattern of disk 28relative to the pattern of disk 30 such that when the opaque segment 46of disk 28 is aligned with the opaque segment 46 of disk 30, andsimilarly the transparent segments 48 of disk 28 are aligned with thetransparent segments 48 of disk 30, a maximum intensity of light reachesdetector 40. Accordingly, whether a polarizer disk such as disk 42 isutilized or whether some other form of disk with a periodic format suchas that of disk 44 of FIG. 3 is utilized, the resulting modulation ofthe light impinging upon detector 40 is (if disk 28 is stationary)periodic. When either disk 42 or disk 44 is utilized, the modulationfrequency is clearly related to the difference between the rotationrates of disk 28 and disk 30, and the phase angle of the modulation isrelated to the difference between the phase an gles of disk 28 and disk30, or accordingly, the difference between the phase angles of source 22and source 24.

The detector 40 (preferably a photocell or phototransistor) isresponsive to the intensity of the beam of light 32 and provides anelectrical signal having a modulation corresponding to the modulation ofthe light. For example, when the polarizer disks 42 are utilized, inwhich case the light intensity varies as the square of the cosine of theangle between the polarization directions of the two disks, the detectorsignal comprises a well-known DC (or steady state) term and a doublefrequency sinusoid, that is, the frequency of the sinusoidal componentof the detector signal is double the rotation rate of disk 30 relativeto disk 28. This can be readily visualized since the intensity of thelight impinging on detector 40 attains two maxima and two minima duringeach rotation of disk 30 relative to disk 28. Since there is essentiallyno time drift of the detector signal relative to the light modulation,the detector signal is synchronized with the light modulation, that is,the phase angle of the sinusoidal component of the detector signal isconstant or locked relative to a reference point on the waveform of themodulated light. And similarly, as will become apparent from an exampleof the operation described hereinafter, the waveform of the modulatedlight is phase locked to the rotations of disks 28 and 30 and sources 22and 24.

An amplifier 50 connects with the output of the detector 40 foramplifying the detector signal to a sufficient value to drive the load26 connecting with the output of amplifier 50. A switch 52 isconveniently provided to disconnect amplifier 50 from detector 40 inorder to enable alignment, to be described below, of the phase angle ofthe load 26 with the phase angles of source 22 and source 24. AmplifierS is AC coupled, for example, capacitor coupled, in order to pass onlythe alternating components of the modulated light, and to reject the DCor steady component of the modulated light. Accordingly, the outputsignal of amplifier 50 is an AC signal (a sinusoid when the polarizingdisks 42 are utilized) which is phase locked to the difference betweenthe phase angle of source 22 and the phase angle of source 24.

The load 26 is typically phase locked to the output signal of theamplifier 50 and may include a mechanical or electrical device such as acounter (not shown) responsive to the AC signal of the amplifier 50 forindicating the difference in rotations of the sources 22 and 24.

The invention is readily utilized for positioning a PP] display on boarda ship in which case the load 26 includes the deflection yoke 54 of thePP] display, the deflection yoke 54 being driven by a synchronous motor56 through gear train 58. In practice gear train 58 comprises typicallya pinion (not shown) connecting directly with the synchronous motor 56and meshing with the teeth of an annular gear (not shown) concentricwith the deflection yoke 56. As is well known, the rotation of thesynchronous motor 56 is locked to the phase of its input signal, hereinthe AC signal of amplifier 50, and accordingly, the rotation imparted tothe deflection yoke 54 is phase locked to the rotations of source 22 andsource 24. It will be shown, hereinafter, that is this embodiment theelectrical signal energizing the synchronous motor 56 has a nominalfrequency of 601-12 and, accordingly, a synchronous motor operating overa frequency range of typically 25-901-12 is utilized to provideoperation over a range of antenna rotation rates.

In shipboard operation, source 22 comprises a gyrocompass 60 forproviding a stable indication of north, a synchro 62 having statorwindings connecting, in a well-known way, to the gyrocompass 60, and agear train 64 interconnecting the rotor shaft (not shown) of synchro 62with the disk 28. Gyrocompass 60, by way of example, has a conventionalratio, 36:1 step up, whereby angle data transmitted electrically fromthe gyrocompass 60 to the synchro 62 is multiplied by a factor of 36.Thus, during one full turn (360) by the ship, the magnetic field of thestator windings of syn chro 62, as well as the rotor shaft, rotates 36full turns.

In shipboard operation, source 24 includes radar equipment comprising anantenna 66 driven by a motor 68 through gear train 70. In this examplethe shaft of motor 68 rotates at 1,800 revolutions per minute (RPM), theantenna 66 rotates at 20 RPM and the gear train 70 has a step down ratioof :1. Motor 68 and synchro 62 are positioned proximately to modulator20 to facilitate a direct mechanical connection indicated by dashed line72, between motor 68 and disk 30, and a similar direct mechanicalconnection, indicated by dashed line 74, between gear train 64 and disk28.

As shown on the diagram of FIG. 1, motor 68 of source 24 and disk 30rotate at 1,800 RPM, and rotate through 90 cycles of revolution for eachrevolution of antenna 66. The same scale factor is utilized for bothsource 22 and source 24, and accordingly, gear train 64 has a step upratio of 2.511 to rotate disk 28 through 90 cycles of revolution foreach revolution of the ship about its yaw axis.

Gear train 58 has a ratio such that deflection yoke 54 rotates at thesame rate as does antenna 66 when the ships heading is constant, theratio being calculated as follows: When the ships heading is constant,disk 28 is stationary. Disk 30 rotates at 1,800 RPM (relative to theship) irrespectively of turning movements by the ship. The intensity ofthe light incident upon detector 40 is modulated at a rate equal todouble the rotation rate of disk 30 when disk 28 is stationary, andaccordingly, the AC signal of amplifier 50 has a frequency of twice therotation rate of disk 30 relative to disk 28, which in this example is 2X 1,800 3,600 RPM or 60l-lz.

The rotor of synchronous motor 56 makes one rotation for each period ofthe AC signal of amplifier 50. Gear train 58 is therefore given areduction ratio of 180:1 so that the deflection yoke 54 rotates at RPM,the rate of rotation of the antenna 66, when the ships heading isconstant.

The operation during a change in ships heading is demonstrated asfollows: Assume, for example, that the ship makes one full turn (360)during a period of time when the antenna 66 rotates relative to the shipa total of N revolutions. The total number of antenna revolutionsrelative to a fixed reference is then N i 1, the plus sign indicating arotation of the ship in the same direction as the rotation of theantenna and the minus sign indicating a rotation of the ship in theopposite direction from the rotation of the antenna. The total numer ofrevolutions of disks 28 and 30 relative to each other is, accordingly,90(N i 1 The total number of periods of the light modulation detectedbydetector 40, the AC signal of amplifer 50, and the rotation ofsynchronous motor 56 is equal to 90(N i" l )2. Gear train 58 reducesthis number by a factor of 180 giving N :t l rotations of the deflectionyoke 54, which is equal to the number of rotations of the antennarelative to a fixed reference.

To initially align the deflection yoke 54 in correspondence with theorientation of the antenna 66, switch 52 is operated to momentarilydisconnect amplifier 50 so that the AC signal of amplifier 50 ismomentarily interrupted from energizing the synchronous motor 56. Thesynchronous motor 56 then begins to slow down until such time as switch52 is operated to reconnect amplifier 50. Successive operations ofswitch 52 allow the antenna 66 to rotate relative to the deflection yoke54 to bring the orientation of the antenna 66 into correspondence withthe orientation of the deflection yoke 54.

It is understood that the above-described embodiments of the inventionare illustrative only, and that modifications thereof will occur tothose skilled in the art. For example, a beam of radiation other thanlight, such as microwave radiation, can be utilized with suitableshuttering devices. Accordingly, it is desired that the invention is notto be limited to the embodiments disclosed herein but is to be limitedonly as defined by the appended claims.

I claim:

1. In a vehicle having means for rotating the vehicle through a headingangle relative to north, a gyrocompass indicating the heading anglerelative to north, an antenna, a drive mechanism for rotating theantenna through a bearing angle relative to the vehicle, and a displayhaving a rotatable indicator of direction, the improvement being astabilization means for positioning the rotatable indicator of directionrelative to the antenna and the vehicle to form a north stabilizeddisplay, the stabilization means comprising:

a radiant source providing a beam of radiation;

modulation means responsive to the heading angle of the vehicle andresponsive to the bearing angle of the antenna for modulating theintensity of the beam of radiation with a modulation frequencyproportional to the rate of rotation of the vehicle through the headingangle and the rate of rotation of the antenna through the bearing angle,and providing a signal having a frequency equal to the modulationfrequency; and

synchronized drive means responsive to the frequency of the signal ofthe modulation means and connecting with the rotatable indicator wherebythe rotatable indicator is positioned.

2.The stabilization means of claim 1 wherein the synchronized drivemeans comprises:

a synchronous motor energized by the signal of the modulation means androtating at a frequency proportional to the frequency of the signal ofthe modulation means; and

means connecting the synchronous motor to the rotatable indicatorwhereby the rotatable indicator is positioned.

3. The stabilization means of claim 2 wherein the modulation meanscomprises:

a first and a second rotatable shuttering means positioned within andtransversely to the beam of radiation, and characterized by a radiationtransmissivity such that rotation of the second shuttering meansrelative to the first shuttering means modulates the intensity of thebeam of radiation with a modulation frequency proportional to the rateof rotation of the second shuttering means relative to the firtshuttering means;

means for rotating the first shuttering means through an angleproportional to the heading angle of the vehicle;

means for rotating the second shuttering means through an angleproportional to the bearing angle of the antenna; and

means for detecting the modulated beam of radiation to provide a signalhaving a frequency equal to the modulation frequency.

4. The stabilization means of claim 3 wherein the first and the secondshuttering means comprise opaque and transparent segments.

5. In a vehicle having an antenna, a drive mechanism for rotating theantenna through a bearing angle with respect to the vehicle, agyrocompass indicating the heading angle of the vehicle relative tonorth, and a display having a rotatable indicator of direction, theimprovement being a stabilization means for positioning the rotatableindicator of direction relative to the antenna and the vehicle to form anorth stabilized display, the stabilization means comprising: a lampproviding a beam of light; a first and a second rotatable polarizer diskpositioned within and transversely to the beam of light;

first means for rotating the first polarizer disk through an angleproportional to the heading angle of the vehicle;

second means for rotating the second polarizer disk through an angleproportional to the bearing angle to the antenna relative to thevehicle, whereby the rotation of the second polarizer disk relative tothe first polarizer disk modulates the intensity of the beam of lightwith a modulation frequency proportional to the rate of rotation of thesecond polarizer disk relative to the first polarizer disk;

a detector positioned to receive the modulated beam of light, thedetector producing a signal having a frequency equal to the modulationfrequency;

a synchronous motor energized by the detector signal and rotating at afrequency proportional to the fre quency of the detector signal; and

means connecting the synchronous motor to the rotatable indicatorwhereby the rotatable indicator is positioned.

6. In combination:

a radiant source providing a beam of radiation;

a first and a second shuttering means disposed within said beam ofradiation for producing a modulation of said beam of radiation at a rateproportional to a frequency of shuttering by said first shuttering meansand to a frequency of shuttering by said second shuttering means; and

output means responsive to said modulated radiation for providing anoutput signal at a rate proportional to said modulation rate, saidoutput means comprising detection means responsive to said radiation forproviding electrical signal pulses at a frequency equal to saidmodulation rate, said output means further comprising means forproviding an angular increment in response to individual ones of saidelectrical signal pulses.

7. in combination:

a radiant source providing a beam of radiation;

a first and a second shuttering means disposed within said beam ofradiation for producing a modulation of said beam of radiation at a rateproportional to a frequency of shuttering by said first shuttering meansand to a frequency of shuttering by said second shuttering means; and

output means responsive to said modulated radiation for providing anouput signal at a rate proportional to said modulation rate, said outputmeans comprising detection means responsive to said radiation forproviding an electrical signal at a frequency equal to said modulationrate, said output means further comprising motor means responsive to theelectrical signal of said detection means for providing said outputsignal, said output signal being a mechanical movement by said motormeans.

8. The combination according to claim 7 wherein said mechanical movementis a shaft rotation synchronized to said modulation.

9. The combination according to claim 8 further comprising a first and asecond drive means for driving respectively said first and said secondshuttering means.

10. The combination according to claim 9 wherein said first drive meansrotates said first shuttering means about an axis of said beam at a rateproportional to a rate of a first rotation, and said second drive meansrotates said second shuttering means about said axis of said beam at arate proportional to a rate of a second rotation.

11. The combination according to claim 10 further comprising a displaydriven by said rotating shaft to indicate a difference in rotationbetween said first and said second rotations.

12. The combination according to claim 11 wherein said first drive meansis responsive to the rotation of a vehicle relative to a compass, andsaid second drive means is responsive to the rotation of an antennarelative to said vehicle.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,766,557 Dated October 16, 1973 Inventor(s) Albert S. Ballard It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

On the title page, the inventor's name should read Albert S.

Ballard not "Albert S. Dallard" Signed and sealed this 16th day of April1971 (SEAL) Attest:

IJI'JWARIJ M.ELI-ZTCIIEII,J1L C. I IA'RSIIALL DANE? Attesting OfficerCommissioner of Patents FORM PO-1050 (10-69) USCOMM DC eowednag u.s.GOVERNMENT PRINTING orncs: was o-ass-su UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No. 3 766, 557 Dated October 16 1973Invent fl Albert S. Ballard It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

On the title page, the inventor's name should read Albert S.

Ballard not "Albert S. Ballard" Signed and sealed this 16th day of April1971 (SEAL) Attest:

EDWARD MJ LETCIIEFQJII. C. I IARSHALL DANA Attesting OfficerCommissioner of. Patents FORM PO-IOSO (10-69) uscoMM-oc 60376-P69 UISIGOVERNMENT PRINTING OFFICE: I969 0-4864!

1. In a vehicle having means for rotating the vehicle through a headingangle relative to north, a gyrocompass indicating the heading anglerelative to north, an antenna, a drive mechanism for rotating theantenna through a bearing angle relative to the vehicle, and a displayhaving a rotatable indicator of direction, the improvement being astabilization means for positioning the rotatable indicator of directionrelative to the antenna and the vehicle to form a north stabilizeddisplay, the stabilization means comprising: a radiant source providinga beam of radiation; modulation means responsive to the heading angle ofthe vehicle and responsive to the bearing angle of the antenna formodulating the intensity of the beam of radiation with a modulationfrequency proportional to the rate of rotation of the vehicle throughthe heading angle and the rate of rotation of the antenna through thebearing angle, and providing a signal having a frequency equal to themodulation frequency; and synchronized drive means responsive to thefrequency of the signal of the modulation means and connecting with therotatable indicator whereby the rotatable indicator is positioned. 2.The stabilization means of claim 1 wherein the synchronized drive meanscomprises: a synchronous motor energized by the signal of the modulationmeans and rotating at a frequency proportional to the frequency of thesignal of the modulation means; and means connecting the synchronousmotor to the rotatable indicator whereby the rotatable indicator ispositioned.
 3. The stabilization means of claim 2 wherein the modulationmeans comprises: a first and a second rotatable shuttering meanspositioned within and transversely to the beam of radiation, andcharacterized by a radiation transmissivity such that rotation of thesecond shuttering means relative to the first shuttering means modulatesthe intensity of the beam of radiation with a modulation frequencyproportional to the rate of rotation of the second shuttering meansrelative to the first shuttering means; means for rotating the firstshuttering means through an angle proportional to the heading angle ofthe vehicle; means for rotating the second shuttering means tHrough anangle proportional to the bearing angle of the antenna; and means fordetecting the modulated beam of radiation to provide a signal having afrequency equal to the modulation frequency.
 4. The stabilization meansof claim 3 wherein the first and the second shuttering means compriseopaque and transparent segments.
 5. In a vehicle having an antenna, adrive mechanism for rotating the antenna through a bearing angle withrespect to the vehicle, a gyrocompass indicating the heading angle ofthe vehicle relative to north, and a display having a rotatableindicator of direction, the improvement being a stabilization means forpositioning the rotatable indicator of direction relative to the antennaand the vehicle to form a north stabilized display, the stabilizationmeans comprising: a lamp providing a beam of light; a first and a secondrotatable polarizer disk positioned within and transversely to the beamof light; first means for rotating the first polarizer disk through anangle proportional to the heading angle of the vehicle; second means forrotating the second polarizer disk through an angle proportional to thebearing angle to the antenna relative to the vehicle, whereby therotation of the second polarizer disk relative to the first polarizerdisk modulates the intensity of the beam of light with a modulationfrequency proportional to the rate of rotation of the second polarizerdisk relative to the first polarizer disk; a detector positioned toreceive the modulated beam of light, the detector producing a signalhaving a frequency equal to the modulation frequency; a synchronousmotor energized by the detector signal and rotating at a frequencyproportional to the frequency of the detector signal; and meansconnecting the synchronous motor to the rotatable indicator whereby therotatable indicator is positioned.
 6. In combination: a radiant sourceproviding a beam of radiation; a first and a second shuttering meansdisposed within said beam of radiation for producing a modulation ofsaid beam of radiation at a rate proportional to a frequency ofshuttering by said first shuttering means and to a frequency ofshuttering by said second shuttering means; and output means responsiveto said modulated radiation for providing an output signal at a rateproportional to said modulation rate, said output means comprisingdetection means responsive to said radiation for providing electricalsignal pulses at a frequency equal to said modulation rate, said outputmeans further comprising means for providing an angular increment inresponse to individual ones of said electrical signal pulses.
 7. Incombination: a radiant source providing a beam of radiation; a first anda second shuttering means disposed within said beam of radiation forproducing a modulation of said beam of radiation at a rate proportionalto a frequency of shuttering by said first shuttering means and to afrequency of shuttering by said second shuttering means; and outputmeans responsive to said modulated radiation for providing an ouputsignal at a rate proportional to said modulation rate, said output meanscomprising detection means responsive to said radiation for providing anelectrical signal at a frequency equal to said modulation rate, saidoutput means further comprising motor means responsive to the electricalsignal of said detection means for providing said output signal, saidoutput signal being a mechanical movement by said motor means.
 8. Thecombination according to claim 7 wherein said mechanical movement is ashaft rotation synchronized to said modulation.
 9. The combinationaccording to claim 8 further comprising a first and a second drive meansfor driving respectively said first and said second shuttering means.10. The combination according to claim 9 wherein said first drive meansrotates said first shuttering means about an axis of said beam at a rateproportional to a rate of a first rotation, and said second drive meansrotates said second shuttering means about said axis of said beam at arate proportional to a rate of a second rotation.
 11. The combinationaccording to claim 10 further comprising a display driven by saidrotating shaft to indicate a difference in rotation between said firstand said second rotations.
 12. The combination according to claim 11wherein said first drive means is responsive to the rotation of avehicle relative to a compass, and said second drive means is responsiveto the rotation of an antenna relative to said vehicle.